1. Field of the Invention
The present invention relates generally to solid state image sensing devices, and more particularly to a solid state image sensing device used as the image sensing element in, for example, a color video camera, according to a frequency interleave system and operable in the field storage mode.
2. Description of the Prior Art
A solid state image sensing element in a known solid state image sensor of a frequency interleave system operable in the field storage mode, is formed by arranging a plurality of fundamental image sensing areas or regions 1, each of which is formed of four rows and 2 columns of picture element portions A.sub.11, A.sub.2l, A.sub.3l, A.sub.4l, A.sub.l2, A.sub.22, A.sub.32, and A.sub.42, as shown in FIG. 1. Columns (vertical direction) and rows (horizontal direction) are shown. The picture element portions A.sub.11 to A.sub.42 have color filters of predetermined spectral characteristics arranged thereon to allow the generation of color charge signals, each corresponding to the amount of light of a selected and predetermined wavelength of an optically picked-up image. In this kind of known solid state image sensor of frequency interleave system, operable in the field storage mode, the filters for the picture element portions A.sub.11 and A.sub.3l in the first column and in the first and third rows are made yellow, as, for example, shown in FIG. 2A. In other words, the color filters of the picture element portions A.sub.11 and A.sub.3l are selected to have such spectral characteristics as to allow light of green and red wavelengths to pass therethrough as illustrated in FIG. 3A. The picture element portions A.sub.2l and A.sub.42 in the first column, second row, and in the second column, fourth row are made cyan color picture element portions. In other words, the color filters of the picture element portions A.sub.2l and A.sub.42 are selected to have such spectral characteristics as to allow light of blue and green wavelengths to pass therethrough as illustrated in FIG. 3B. Further, the picture element portions A.sub.4l and A.sub.l2, A.sub.22 and A.sub.32 in the first column, fourth row, and in the second column, first to third rows, are made green color picture element portions. In other words, the color filters of the picture element portions A.sub.4l, A.sub.l2, A.sub.22, and A.sub.32, are selected to have such spectral characteristics as to allow light of green wavelength to pass therethrough as illustrated in FIG. 3C.
Then, two adjacent picture element portions in the column direction (the vertical direction as shown in FIGS. 1 and 2) of the picture element portions A.sub.11 to A.sub.42 of the fundamental image sensing region 1 are combined to form a set of picture elements, from each of which is generated a charge signal ased on the optical image formed thereon.
Reference will be made to the example of FIG. 2A. In the first field, from the picture element portions A.sub.11, A.sub.2l, A.sub.l2, and A.sub.22 in the first and second rows, there is derived a modulation component R+B on the n-th line and from the picture element portions A.sub.31, A.sub.4l, A.sub.32, and A.sub.42 in the third and fourth rows, there is derived a modulatation component R-B on the (n+1)th line. That is a yellow pick-up or video signal from the picture element portion A.sub.11 in the first column, first row, having color component signals (G+R) of both green and red, and a cyan video signal from the picture element portion A.sub.21 therebeneath, having color component signals (B+G) of both blue and green, are mixed together to yield a sum signal of the signals, expressed as S.sub.1 =(G+R)+(B+G)=R+B+2G. In like manner, video signals G of green derived from two vertically-adjacent green picture element portions A.sub.l2 and A.sub.22 in the second column and its first and second rows are added together, thus yielding a sum signal expressed as S.sub.2 =G+G=2G. The subtraction of these signals S1 minus S2 results in a modulation component, S.sub.1 -S.sub.2 =(R+B+2G)-2G=R+B on the n-th line. In the same way, a yellow signal (G+R) is derived from the picture element portion A.sub.3l in the first column, third row, and is mixed with a green signal G derived from the green picture element portion A.sub.4l therebeneath, thus yielding a sum signal expressed as S.sub.3 =(G+R)+G=R+2G. A green signal G is derived from the green picture element portion A.sub.32 in the second column third row, and mixed with a cyan signal (B+G) derived from the picture element portion A.sub.42 therebeneath, thus yielding a sum signal expressed as S.sub.4 =G+(B+G)=B+2G. Then, the subtraction of these signals S.sub.3 minus S.sub.4 yields a modulated component, S.sub.3 -S.sub.4 =(R+2G)-(B+2G)=R-B on the (n+1)th line. Two rows of the elements in FIGS. 1-2D form one line of the raster).
In this way, the modulated component (R+B) on the n-th line and the modulated component (R-B) on the (n+1)th line are demodulated into red and blue signals 2R and 2B as the demodulated components of the interleaved signal. The same luminance signal Y=R+B+4G is demodulated on the n-th line by the signals from the picture element portions A.sub.11, A.sub.2l, A.sub.l2, and A.sub.22 and on the (n+1)th line by the signals from the picture element portions A.sub.31, A.sub.4l, A.sub.32, and A.sub.42.
In the second field, the picture element portions A.sub.2l, A.sub.3l ; and A.sub.22, A.sub.32 in the second and third rows are combined to generate a modulated component R+B on the n'-th line. The picture element portions A.sub.4l, A.sub.11 ; and A.sub.42, A.sub.l2 on the fourth row and the first row of the next fundamental image sensing region 1 (not shown) located thereunder are combined to generate a modulated component (R-B) on the (n'+1)th line. Then, the demodulated components 2R and 2B and the luminance signal R+B+4G are obtained similarly.
The demodulated signals are processed by a matrix to produce color difference signals R-Y and B-Y.
FIG. 4 schematically shows an arrangement of a signal processing circuit for such processing. Referring to FIG. 4, there is shown a solid state image sensing element 11 which comprises a large number of the fundamental image sensing regions 1 described above, arranged in columns and rows. The luminance signal Y therefrom is supplied trough a low pass filter 12 and a luminance signal processing circuit 13 for the .gamma.-correction to a color encoder 14. At the same time, a part of the luminance signal Y from the low pass filter 12 is supplied to a matrix circuit 15. Starting with the modulated signal components (R+B) and (R-B) from the solid state image sensing element 11, processing by a demodulating circuit 16, a delay line 17 with a delay time corresponding to one horizontal scanning period (1H), an adding circuit 18 and a subtracting circuit 19, there are produced red and blue signals R and B. These red and blue signals R and B are supplied to the matrix circuit 15, from which are derived the color difference signals R-Y and B-Y, which are then supplied to the color encoder 14. Thus, from its output terminal is produced the video output signals according to the NTSC system.
This solid state image sensor of frequency interleave system, operable in the field storage mode, needs the matrix circuit which generates the color difference signals R-Y and B-Y, since the demodulated components of the interleaved signal are obtained in the form of R and B signals.
This solid state image sensor has the disadvantage of luminance aliasing distortion. Specifically, for a white picked-up optical image formed on the image sensor, the green picture element portions A.sub.l2, A.sub.22, A.sub.32, and A.sub.4l generate only the green signal component G, while the other picture element portions A.sub.11 and A.sub.3l of yellow, or A.sub.2l and A.sub.42 of cyan, generate the green and red signal components (G+R) or blue and green signal components (B+G). As a result, there is a difference in the luminance sensitivity for white light, between a group of the picture element portions A.sub.l2, A.sub.22, A.sub.32, A.sub.4l ; and another group of the other picture element portions A.sub.11, A.sub.2l, A.sub.3l, A.sub.42, resulting in the luminance aliasing distortion corresponding to the arrangement pattern of the respective picture element portions. This causes the quality of the resulting picture to be deteriorated.
FIG. 2B shows another arrangement of the fundamental image sensing region 1 of the known image sensor of this kind. Referring to FIG. 2B, the picture element portions A.sub.11, A.sub.2l, A.sub.3l, and A.sub.42 are made white picture element portions, or the color filters thereof are made to have such spectral characteristics as to allow all light of blue, green and red wavelengths to pass therethrough, as shown in FIG. 3D. The picture element portions A.sub.4l and A.sub.22 are made yellow picture element portions and the picture element portions A.sub.l2 and A.sub.32 are made cyan picture element portions. Similarly to the manner described above, for example, the modulated component (R+B) is generated on the n-th and n'-th lines and the modulated component (R-B) on the (n+1)th and (n'+1)th lines. Then, the addition and subtraction of these modulated components yield the demodulated components 2R and 2B of the interleaved signal and a component (3R+4G+3B) as the luminance signal Y.
Also in this case, the same matrix circuit as shown in FIG. 4 must be provided, in order to obtain the required color difference signals. Further, since there is a large difference in the luminance senstivity for the white light among the picture element portions A.sub.l2, A.sub.32 of cyan, A.sub.22, A.sub.4l, of yellow and A.sub.11, A.sub.2l, A.sub.3l, A.sub.42 of white, a luminance aliasing distortion occurs.
FIG. 2C shows still another example of the fundamental image sensing region 1. Referring to FIG. 2C, the picture element portions A.sub.11, A.sub.3l, and A.sub.4l are made as cyan picture element portions (B+G), the picture element portion A.sub.2l as the white picture element portion (B+G+R), the picture element portions A.sub.l2, A.sub.22, and A.sub.32 as green picture element portions (G), and the picture element portion A.sub.42 as the yellow picture element portion (G+R). In this case, a modulated component (2B+R) is generated on the n-th and n'-th lines and a modulated component (2B-R) on the (n+1)th and (n'+1)th lines. The addition and the subtracation of these modulated components yield demodulated components 4B and 2R of the interleaved signal and a luminance signal (R+3G+2B). Also in this case, the same matrix circuit as shown in FIG. 4 must be provided. Further, a luminance aliasing distortion is caused from a difference in the luminance sensitivity to white light, among the picture element portions.
FIG. 2D shows a further example of the fundamental image sensing region 1. The picture element portins A.sub.11, A.sub.2l, A.sub.3l, are made as white picture element portions (B+G+R), the picture element portion A.sub.4l as cyan picture element portion (B+G), the picture element portion A.sub.22 as green picture element portions (G), and the picture element portions A.sub.l2, A.sub.32 and A.sub.42 as yellow picture element portions (G+R). In this case, a modulated component (2B+R) is generated on each of the n-th and n'-th lines and a modulated component (2B-R) on each of the (n+1)th and (n'+1)th lines. The addition and subtraction of these modulated components yield demodulated components 4B and 2R of the interleaved signal and a luminance signal (3R+4G+2B). Also in this example, the matrix circuit of FIG. 4 is necessary, and luminance aliasing distortion occurs.
The known solid state image sensors of an interleave system; operable in the field storage mode, requires the following requirements (i) to (vi).